I didn't notice this thread before, but I was thinking of making an Altoids tin-sized PCB for the OBD MPGuino, and looking at the code for the OBD one and comparing the schematic of the mainline one, there doesn't seem to be much of a reason that both designs can't share a PCB. Maybe we should look into making a unified design and make a volume order?
(Personally, the OBD one is very appealing to me, since I have two OBD-II enabled cars...)